This invention relates to a grounding system for electrical distribution systems and, more particularly, to a grounding system which develops a compensating voltage which reduces the stray voltage on the grounded neutral conductor to a value approaching, or more nearly equal to, true earth ground potential.
High-voltage distribution systems in rural, commercial and residental areas are normally connected to the user by way of a step-down service transformer the secondary of which is connected to a service entrance. Typically, the secondary of the transformer in a single phase system provides a pair of live or "hot" conductors and a center-tapped, neutral conductor. The neutral conductor is connected to the grounding terminal block at the service entrance. The grounding terminal block is then grounded in proximity to the service entrance by way of a ground electrode or ground rod in the earth. These grounding circuits are inherently resistive due to the resistivity of the soil, the finite resistance in the secondary neutral conductor, the ground rod cable and the grounding electrode itself. Any current which flows through the resistance of the grounding circuit develops a so-called "stray voltage" across the resistance which is greater than "0"--i.e., the value of true earth ground potential. The current which develops this stray voltage is attributable to loads on the user's site as well as other loads on the high voltage power distribution system. This stray voltage may reach values that are a hazard to humans and equipment in the case of commercial and residental users, and to both humans and animals in the case of rural or farm users.
This problem is perhaps best exemplified by dairy milking parlor installations wherein the stray voltage adversely affects the milk production and health of the milk cows. It has been reported that a stray voltage of less than 0.5 volts generally does not present a problem to livestock such as cows. A stray voltage in the range of 0.5 to 1.5 volts adversely affects the cows and a stray voltage in excess of 1.5 volts is considered a serious problem. It has been reported that cows which are subjected to the stray voltage on the grounded neutral conductors of the farm electrical distribution system produce less milk. This can subject the cow to a disease known as mastitis which makes the milk unfit for human consumption and can result in the cow's death.
This stray voltage problem is also present in other commercial and residential applications. For example, the stray voltage can deleteriously effect electronic devices such as computers and other sensitive equipment.
In the prior art, the stray voltage problem in dairy barn situations has been dealt with by providing extensive grounding systems such as metallic floor grids which are connected to the grounded neutral terminal. However, this solution presents a so-called "entry problem" wherein the cows are nevertheless subjected to the stray voltage as they step into the barn and onto the grid. Other proposed solutions have included elaborate "super grounds" wherein the resistance to true earth ground has been reduced by using a more extensive grounding circuit. However, these techniques have suffered with inherent limitations on the ability to reduce the magnitude of the neutral-to-earth voltage particularly in installations where the neutral conductor current is relatively high and/or the system impedance is relatively low. That is, with respect to the latter, a low system impedance is more difficult to shunt with additional grounding devices. Both of these prior art systems have the disadvantages that they are difficult and expensive to install.
These and other disadvantages are overcome by the present invention wherein a grounding system is provided which, in effect, provides a simulated ideal ground electrode or an "electronic ground rod" for grounding the neutral terminal of an electrical distribution system.